USP10.dll is the Uniscribe Unicode script processor and it already comes with Windows. Check your c:\windows\system32 folder and you will see that there is already a copy in there (yes, even on XP 64).

The problem is that Avidemux stopped supporting XP, so putting a copy of USP10.dll from Windows 7 won't help because there are other dependencies which will prevent it from working. If you put a copy of USP10.dll from Windows 7 in XP (and patch it to work), then you will get a problem with MSVCRT.DLL. If you fix that, there will be something else and so on.

XP users cannot use Avidemux anymore. You will have to either use an older version (assuming that the required task is compatible with an older version--the old version has the necessary features and does not have bugs that will manifest), or use something else like ffmpeg.